This invention relates to a torque coupling device of the synchronous reluctance type which uses two rotors having a variable rotary shutter of diamagnetic (zero permeability) material to provide a reluctance gradient in a magnetic circuit. The reluctance gradient is the rate of change in circuit reluctance that occurs with respect to a small angular displacement of the two rotors of the coupling. This gradient produces a magnetic torque coupling force between the two rotors. Ideally, there is no sustained energy loss incurred in producing the coupling force. Of course, in the case of a magnetic field, excitation losses are sustained due to current flowing in the excitation coil. These losses may be eliminated by using a permanent magnet or a superconducting coil.
Conventional synchronous reluctance torque couplers, as for example U.S. Pat. Nos. 4,013,384 and 4,115,040, of opposite poles of permanent magnets mounted on two rotors to align to a position of minimum reluctance in a properly arranged magnetic circuit. Others include U.S. Pat. No. 3,890,515 which includes an excitation winding to magnetize salient ferromagnetic polar projections on the two rotors, which polar projections will then tend to align in a minimum reluctance magnetic circuit. For both of these types of torque couplers, the maximum change in magnetic circuit reluctance that can be realized is proportional to the height of the poles as compared to the non-pole space between the poles. The minimum reluctance that can ever be obtained is limited by the unity permeability of the space occupied by the non-poles.
A unique feature of a synchronous reluctance coupling is that it maintains a precise angular displacement between the two magnetically coupled rotors, the driver leading the driven rotor in phase by a few degrees depending on the torque and the field. The angular displacement, or phase angle, between the two rotors is independent of the speed of the rotors. It is zero when there is no torque on the driven shaft. There is a precisely discernible phase angle at which a maximum or pullout torque occurs and synchronism between the rotors is lost. The phase angle between zero and the pullout angle increases with the applied torque, but is inversely proportional to the magnetic field. For any given steady-state torque, not exceeding the pullout angle, there is no energy expenditure in the rotors, mechanical losses due to windage and bearing friction excepted. If synchronism is lost, a pulsating torque with an average value of zero is developed.
A distinction should be noted between synchronous reluctance couplings and couplings based on either the hysteresis principle or the induction principle. These other couplings develop torque as a consequence of energy losses, i.e., heating, in one of the rotors. The torque is proportional to the quotient of the rotor loss divided by the slip speed. The induction, or eddy current coupling cannot develop torque when the rotors are running synchronously. The hysteresis coupling can develop a reduced torque at synchronism; however the precise phase relationship between the rotors is indeterminant, depending on the previous torque history of the device.
The objective of the reluctance coupling designer is to provide a construction that achieves a maximum reluctance gradient with the most economical use of space and materials. If large ferromagnetic rotor poles can be replaced with thin, light-weight flux shutters, then a much improved product would result.